The coefficient of the hydric expansion of a composite structure, usually denoted by .beta., is determined by the equation: ##EQU1## where L.sub.O and M.sub.O respectively represent the initial length and initial weight of a dry sample of the composite structure, .DELTA.L and .DELTA.M representing the evolution of these quantities when this sample is impregnated with water.
Knowledge of the coefficient of the hydric expansion .beta. is of special interest in space applications, having regard to the fact that the embarked optics are frequently supported by composite structures. In fact, the space vacuum has the effect of draining these composite structures and the release of the water intially contained in these structures is expressed by dimensional variations which are recovered on the optics they support.
Currently, these dimensional variations are taken into account by electro-mechanical devices associated with the embarked optics and which make it possible to correct the positioning of the latter by displacements along the optical axis. However, control of the entire bearer structure and determination of the adjustment sizes of the optics immediately before launching requires a full knowledge of the reaction of the materials when diffused in water.
Furthermore, given the fact that the dimensional variations resulting from the draining under vacuum of an initially humid composite structure are extremely small (sometimes less than 1 .mu.m per mm of sample) and slow (several weeks), there currently exists no sufficiently accurate and stable dilatometer able to measure these variations to be taken into account during the lifetime of the composite parts supporting the embarked optics.
The installing of such a device has up until now come up against difficulties mainly linked to the fact that existing displacement transducers, which seemed sufficiently accurate and stable so as to be able to be used, were sensitive to the evolution of the ambient humidity in proportions equal to at least the proportions of the measurements to be made.
If it appeared to be possible to resolve this problem by placing the transducer outside the zone containing the sample and in which the ambient humidity is made to vary, this would result in other errors on account of the ensuing need to place a linking member between the transducer and the sample. In fact, this linking member would have traversed a transition zone subjected to the original thermic stresses necessarily resulting in non-compensated deformations and local test environment disturbances.